The invention relates to an illuminating system for producing a condensed light usable in the production of prints on variable contrast photosensitive material from a negative by exposure of the latter to said condensed light, which system comprises a light source, plate means having an aperture and being placed next to said light source and permitting light from said light source to be directed toward said negative in the form of a light cone, and an elongated two-colour filter strip of light-transmitting material, having a longitudinal axis and a frontal strip face turned toward said light source, and an opposite face; at least one of said frontal and opposite faces thereof bearing three transverse zones adjacent one another and disposed in sequence along said strip axis to constitute two end zones and a middle zone therebetween, a first one of said end zones being a first-colour end zone adapted for transmitting light in a first of two mutually exclusive wavebands, the other end zone being a seond-colour end zone adapted for transmitting light in the second one of said two wavebands, and said middle zone bearing a plurality of discrete first colour elements transmitting light in said first waveband and a plurality of discrete second colour elements which transmit light in said second waveband.
The invention further relates to a two-colour filter strip of the aforesaid type. The novel illuminating system and/or the novel filter strip can be used particularly in a lamphouse for photographic enlargers.
An illuminating system of this type is described in British patent specification No. 2,023,297, in which the two-colour filter strip bears two zones next to one another, along the longitudinal strip axis, which are separated by a sharp transverse junction between the two zones transmitting light in different mutually exclusive wavebands, in particular yellow and magenta.
However, even when, as in FIG. 4a of that GB-PS, there is a middle zone having discrete areas as described hereinbefore, the partial sums of the yellow-transmitting surface areas of the middle zone illuminated through the aperture in the plate means are not progressively increasing, but are alternatingly increasing and again decreasing while the strip is scanned through the said aperture.
Moreover, it has been found that, there is no satisfactory progressive mixing of light, but special light mixing means must be provided.
It is also known to compensate alterations of the effective filter density resulting from adjustment of the filter by corresponding but opposite adjustment of a disphragm. This arrangement employs filters of continuously variable density, which are expensive.
It is also known to obtain a continuously adjustable filter by using a movable filter with filter material cut into a comb-like array of wedge-shaped elements, and a matching comb-like segment which remains in the optical path. The colour density of such a filter may be continuously varied from approximately zero up to the maximum colour density. The disadvantage of this arrangement is, that six colour filter discs are present in the optical path at the same time and that much light is therefore lost. (viz. col. 1 of U.S. Pat. No. 3,418,048).
In U.S. Pat. No. 3,418,048 to Anton Veit, there is disclosed a filter assembly consisting of two filter discs each of which bears three comb-like arrays of wedge-shaped colour filter elements, e.g. of yellow, magenta and cyan colour, which are mutually displaced by 120.degree. and each in part overlaps the two adjacent arrays, while the spaces intermediate the wedge-shaped comb teeth and the center of the filter disc remain colourless, through which center the optical path of unfiltered light extends when the two filter discs are completely superimposed. The discs are arranged to gyrate about the optical axis without rotation about their own axes, thus introducing each peripheral portion in turn of the filter discs into the optical path to vary the filter colour. A maximum filtering effect is achieved when the peripheral portion of one of three filtering zones of one filter disc is superimposed upon the corresponding peripheral portion of the other disc.
This rather complicated known structure serves to vary the colour density of the filter from approximately zero up to the maximum colour density.
But while the loss of light is reduced compared with the last-mentioned earlier adjustable filter arrangement with six colour filters, there is still loss of filtered light and the whole apparatus is complicated and correspondingly expensive.
Similar drawbacks are found in the colour correction filter apparatus of Keith Ashton, described in U.S. Pat. No. 3,536,402 which requires the use of three separate filter discs. Ashton teaches that the light after filtering should be highly diffused in order completely to mix the filtered light with unfiltered light, involving again considerable loss of light.
The interference filter described by Ashton consists of a thin interference film which is so deposited upon a base plate that it forms coated and uncoated areas which are small in relation to the total area of the base plate, the coated (or uncoated) areas being surrounded by uncoated (or coated) areas, the coated areas serving to reflect the undesired wavelengths and transmit substantially all other wavelengths of the exposing light beam of the said apparatus and the uncoated areas serving to transmit the exposing light beam substantially unchanged.
In this way the light transmitted through the interference filter is a mixture of unchanged light transmitted through the uncoated areas and colour corrected light transmitted through the coated areas. The ratio of the total uncoated to coated area of the filter will determine the overall density of the filter and may either be constant throughout the area of the filter or may change progressively through the length of the filter so that the filter may be moved across the exposing light beam for adjustment of its effective density. The coated areas desirably give a heavy or substantially complete suppression of light of the wavelengths that it is desired to suppress.
The configuration of the uncoated and coated areas may be in any form but dots similar to the pattern of a half-tone printed picture are usually quite suitable.
Finally, Robert W. C. Hunt described in U.S. Pat. No. 3,089,386, a filter pair for controlling the spectral composition of a light source, which is made in the form of dye or glass wedges 5 and 6, in which the absorption gradually increases from one end to the other and these wedges can be moved across the light beam from a light source 7 so as to give different degrees of absorption. For example, by attaching to a cyan coloured filter wedge 5 a correcting filter wedge 6 is wedged in the opposite direction, it is possible by careful choice of the colour and density of the constituent wedges to obtain a composite wedge in which only the red absorption varies from one end to the other, the green and blue absorptions remaining constant. Movement of such a composite wedge 5, 6 in the light beam thus enables its red content to be varied independently of its green and blue contents and similar composite wedges can be constructed to provide independent control of the green and blue contents of the beams.
It is further suggested that there is used with each filter-pair, a short compensating wedge-pair which is similar to a section of the light end of each coloured wedge 5 and 6, respectively, but mounted with the wedgings in the opposite direction so that the device does not have to be used out of focus with respect to the final image obtained. The compensating wedges are stationary and are sufficiently large to extend completely across the path of the light beam as defined by an apertured plate 10.
Summing up therefore, in none of the prior art proposals is there described a light filter systemin which movement of the filter past the light source modulates the light passing the filter in two mutually exclusive wavebands in a complementary manner. Thus in none of the prior art proposals is there the need to obviate any sharp transverse junction between two zones transmitting light in different mutually exclusive wavebands. Therefore none of the prior art proposals could be used to solve the problem which is solved in the illuminating system of the present invention.